This lesson is general education about U.S. personal finance, not financial, tax or investment advice. Tax rules and limits change every year; check IRS.gov or a qualified professional for your situation.
What you’ll learn
- Compounding means earning returns on your past returns, not just on what you put in.
- In a hypothetical example, starting 10 years earlier more than doubles the ending balance.
- The rule of 72 estimates how long money takes to double at a given rate.
- Compounding works against you on debt, especially high-interest credit cards.
Imagine earning interest on your savings, and then earning interest on that interest. Repeat for years and the growth curve bends upward. That’s compounding.
A simple example
Put $1,000 somewhere that earns 6% a year. After one year you have $1,060. In the second year you earn 6% on $1,060, not on $1,000, which gives you $1,123.60. The extra $3.60 is compounding at work. It’s tiny at first and large later.
Starting early versus starting later
Try the math. Two savers each put away $200 a month, and in this hypothetical both earn 6% a year, compounded monthly.
- Saver A starts at 25 and contributes for 30 years: $72,000 in total, growing to about $200,900.
- Saver B starts at 35 and contributes for 20 years: $48,000 in total, growing to about $92,400.
Saver A put in $24,000 more but ended with about $108,500 more. The extra decade did most of the work.
Real returns won’t arrive in a smooth 6% every year, but the lesson holds: time in the market is the part you control by starting.
The rule of 72
Divide 72 by an annual rate of return to estimate how many years it takes money to double. At 6%, that’s about 12 years. At 8%, about 9 years. It’s an approximation, but a handy one.
The same math on debt
Compounding doesn’t care whether it’s working for you. Credit card balances grow with interest charged on unpaid interest, often at rates far higher than investments are likely to earn. Paying down high-interest debt is one of the surest “returns” available.
Putting it to work
The easiest way to benefit is to automate saving early in your working life, for example through a 401(k) that takes contributions from each paycheck. Keeping costs low matters too, because fees shrink the amount left to compound; see index funds in plain English.
Common questions
Is a 6% return realistic?
It’s a round number used for illustration, not a forecast. Real investment returns vary from year to year and can be negative, and past returns don’t guarantee future ones.
Does compound interest apply to savings accounts?
Yes. Interest is added to your balance, and then earns interest itself. The rates are usually lower than long-term investment returns, but the money isn’t exposed to market losses.
Where can I run my own numbers?
The SEC’s Investor.gov website has a free compound interest calculator.